The present invention relates generally to laser systems, and more particularly to interferometers for use with laser radar systems.
As is known in the art, interferometers are used in laser radar systems for determining the beat frequency between transmitted laser signals and target-reflected return signals to thereby determine such target parameters as range and Doppler speed. These interferometers are used in both homodyne and heterodyne laser radar systems. In a typical interferometer, a laser produces a linearly polarized (such as p-polarized) single (such as TEM.sub.00) mode beam of electromagnetic energy which is directed through a polarization discriminator, such as a Brewster plate, which couples the p-polarized beam to a quarter-waveplate. The quarter-waveplate transforms the polarization of the beam to circular (such as right-circular) polarization. The circularly polarized laser beam is transmitted toward a target, a portion of the transmitted beam being reflected by the target and returned to the interferometer as an oppositely-circularly (such as left-circularly) polarized beam. The quarter-waveplate transforms the polarization of the return beam to linear polarization orthogonal to the linear polarization of the beam produced by the laser (e.g., to an s-polarized beam). The s-polarized beam is focused on a detecting surface of a detector element. In a homodyne interferometer, a portion of the p-polarized beam produced by the laser is deflected and the polarization thereof rotated (such as by a half-waveplate) to a polarization identical to the s-polarized target reflected return beam focused onto the detector, thereby providing an s-polarized local oscillator (L.O.) beam. The local oscillator beam is also focused on the detecting surface of the same detector element as the target-reflected return beam. The superimposed target-reflected return and L.O. beams are of identical linear polarization and have the same plane wavefronts. The local oscillator beam also has a Gaussian intensity distribution on the detector element, which is derived from the single mode TEM.sub.00 output of the laser. The superimposed signals interfere on the detector element, with the detector element thereby producing a signal which can be processed to yield range and/or radial velocity of the target.
Range measurement using such an interferometer may be accomplished in a typical state-of-the-art C0.sub.2 laser radar system by means of linear up and down frequency chirp combined with coherent detection of the return signal using a time-delayed sample of the chirped transmitter as the local oscillator. The chirp is generated by applying a voltage ramp to a piezoelectric transducer which carries one of the laser mirrors. The difference frequency between the local oscillator and signal beams depends on the range and velocity. Frequency measurements made on the upchirp and the downchirp are added and subtracted to yield the range and velocity. Other suitable modulation formats may also be used, including pulsed.
In such a system, the range resolution is dependent on the chirp rate and the dwell time. For a given piezoelectric transducer and driver with particular velocity vs. time characteristics, the chirp rate of the laser is inversely proportional to the separation of the laser mirrors. That is, chirp rate may be increased by using shorter lasers, which lasers provide a beam of lower output power. There is a need for higher performance C0.sub.2 laser radar sensors for future systems. Both longer range and smaller range resolutions are desired which leads to higher laser power and wider bandwidth. The tradeoff between laser power and chirp rate by varying the laser length is therefore not appropriate for the system improvements needed.